1. Field of the Invention
The invention is directed to an emulator and compensator for polarization mode dispersion with which the polarization of an electromagnetic, preferably optical, wave can be modified frequency-dependent.
The invention is therefore a frequency-dependent polarization transformer at the same time.
2. Description of the Related Art
Long light waveguide transmission links are utilized in optical transmission technology. Conditioned by manufacture, the light waveguides themselves are not completely isotropic but slightly birefringent. A frequency-dependent polarization transformation of the transmission link derives because of the great transmission length. This is called xe2x80x9cpolarization mode dispersionxe2x80x9d or xe2x80x9cpolarization dispersionxe2x80x9d (PMD). This dispersion leads, in particular, to the spreading of transmitted pulses, which limits the usable transmission data rate. The situation is aggravated in that the transmission behavior and, thus, the PMD of the link changes due to temperature or mechanical stressing. Adaptive PMD compensators are therefore required that are inserted into the transmission path. For developing these or simply for testing the PMD tolerance of transmission systems that are not PMD-compensated, adjustable but simple, cost-beneficial and low-attenuation PMD emulators are often used that can simulate the frequency-dependent polarization transmission behavior of LWG links having a length of up to several 1000 km at different points in time and at different temperatures in a statistically significant way.
At at least one optical frequency, the PMD compensator can make the polarization transmission behavior of the overall system of transmission link and compensator (or vice versa, dependent on the arrangement) approximately frequency-independent in approximation of the first and, potentially, higher order as well. The aim in wavelength-division multiplex mode is to achieve this frequency independence at the operating wavelengths. The invention can be utilized both as a PMD emulator as well as a PMD compensator.
Demands made of such an assembly are low insertion attenuation, compatibility with light waveguides and frequency-independently variable polarization transmission behavior in many aspects.
Clearly, an assembly that has the same or, respectively, inverse transmission properties as a transmission link is suitable for the emulation or compensation of PMD. In the literature, PMD is often mathematically described by many retarders or polarization rotators that are arranged between more highly birefringent LWG sections, i.e., LWG sections exhibiting noticeable delay times between the two principal polarizations. These highly birefringent LWG sections maintain or preserve two principal polarizations (principal states of polarization), (PSP), orthogonal relative to one another and are therefore polarization-maintaining light waveguides (polarization-maintaining fibers), (PMF). These PMF are highly polarization dispersive. A corresponding example is described in the Conference Volume of the Optical Fiber Communications Conference 1995 (OFC ""95) of the Optical Society of America as Presentation WQ2 on pages 190-192.
Electron. Lett., Feb. 17, 1994, Volume 30, No. 4, pp. 348-349 describes PMD compensation. Here, too, a plurality of PMF sections that are connected by polarization transformers are employed. This reference is significant because it discloses the connection of a PMD compensator to an optical receiver as well as the acquisition of a control criterion and therefore serves as a genus for this invention. One can see that such arrangements can be employed both as PMD compensators as well as as PMD emulators. In practice, the references are limited to very few PMF sections and the optical attenuation that occurs should probably be rather high due to the required splice connections. Functionally similar or equivalent arrangements that allow a great number of such polarization transformers and PMF sections to be cascaded such that they exhibit a very low attenuation, however, are unknown.
It is therefore an object of the present invention to specify a PMD emulator and PMD compensator that has a very low insertion attenuation compared to the Prior Art and that can also be easily manufactured.
This object is achieved by an apparatus that is an emulator or compensator of polarization mode dispersion of a light wave comprising an input; a light waveguide connected to the input, of which at least one part is a polarization-maintaining light waveguide with a differential group delay time; a polarization transformer connected to the light waveguide; an output connected to the light waveguide; a light waveguide that merges butt-free and splice-free into a polarization-maintaining light waveguide that has a linear birefringence comprising a differential group delay time that can be exposed to a mechanical stressing in a polarization transformer; and a conversion mechanism that can at least partly convert a principal polarization of the polarization-maintaining light waveguide into an other principal polarization orthogonal to the principal polarization with an arbitrary and interruption-free, selectable phase shift relative to the principal polarization.
In the inventive apparatus, the light waveguide may be constructed such that: 1) the light waveguide is twisted between neighboring retaining points that do not enclose a polarization transformer so that the light waveguide acts as a polarization-maintaining light waveguide with circular birefringence, and 2) the light waveguide is not twisted, at least in a specific position of fiber loops, between retaining points enclosing a polarization transformer that comprises at least one movable fiber loop that, given movement, causes a mechanical stressing of the light waveguide, so that the fiber loops act like rotatable wave plates.
The light waveguide may comprise a polarization-maintaining light waveguide with linear birefringence.
The inventive apparatus may contain at least one anchor point of the conversion mechanism comprises a plurality of comb tines, the comb tines belonging to different anchor points being movable relative to one another and following one another in periodic succession, so that torsion sections proceeding therebetween can be twisted in a periodically changing rotational manner.
A plurality of anchor points of the conversion mechanism that are separated from one another by successive torsion sections between two ending anchor points may be used as may a stepping motor for rotating an anchor point of the conversion mechanism.
At least one of the fiber loops can act at least approximately as a rotatable quarter-wave plate.
A plurality of individual emulators or compensators each having a respective input and an output, may be used wherein an output of an individual emulator or compensator can be connected to an input of a following one.
A longer section of polarization-maintaining optical fiber (PMF) is inventively suitable for manufacturing a PMD emulator or PMD compensator. The principal polarizations, as in the case of commercially obtainable, polarization-maintaining optical fibers, are thereby preferably approximately linear in a first exemplary embodiment. Torsion sections in which the PMF twists that result in polarization transformations are distributed over the length.
The torsion of PMF is already known as a mechanism from Applied Optics, Vol. 18, No. 13, pages 2241-2251, as a mechanism with which the polarization transformation can be implemented in linearly birefringent LWGs, see FIG. 9 therein. However, the birefringence of commercially obtainable PMF is so great that a torsion by the 68xc2x0 recited therein would destroy the PMF, at least over the long term.
For use of commercially obtainable PMF, a plurality of torsion sections having alternating torsion direction are therefore inventively cascaded for producing the desired transformation. Alternately, a specific PMF that is more weakly birefringent than commercially obtainable PMF but far more highly birefringent than normal LWG may be provided.
The torsion can be variably fashioned in all of these instances, by, for example, employing stepping motors.
The range of torsion that is naturally limited because of the limited mechanical strength of light waveguides can impeded compensatability, particularly for operation as a PMD compensator. Function imprecisions can also arise due to lengths of the torsion sections that are not ideally defined due to manufacturing factors and similar influences. These potential disadvantages can be eliminated by inserting additional torsion sections. It can thereby be beneficial to arrange a plurality of mutually independently twistable torsion sections or groups of torsion sections close to one another. xe2x80x9cEndless polarization controlsxe2x80x9d are desirable because to as endless polarization controls are desirable. The reason for this is that, particularly, for operation as a PMD compensator, an interruption-free compensation is required; any, interruption (even short) could lead to undesired bit errors in the receiver because of the extremely high transmission bit rates that are usually present. IEEE Journal of Lightwave Technology, Vol. 9, October 1991, No. 10, pages 1353-1366 provides a good overview of endless polarization controls as, in particular, do the references cited within. Endless polarization controls that are based on twistable, polarization-maintaining optical fibers, however, are not yet known.
Instead of linearly birefringent PMF, circularly or elliptically birefringent PMF can also be utilized. However, the polarization transformers would then have to be modified.